The present invention relates to communications systems which use variable-symbol-length coding schemes, such as pulse-width modulation or burst-length modulation.
Many of the innovative teachings set forth herein will be described in the context of a system for short-range wireless data communication between a base station and a portable low-power module. Such systems can be extremely useful in many contexts, such as inventory control for automated manufacturing, control of personnel access to secure facilities, medical monitoring of inpatients, theft control, and others described below. However, although these applications serve to illustrate the advantages of the present invention, the inventive concepts are applicable to a very wide variety of communications systems.
However, such a system is subject to many constraints. First, the maximum power levels for RF (radio-frequency) emissions are stringently limited by law. Second, if the portability of the portable stations is to be maximized, the battery weight must be small, and this means that the power consumption of the portable module-in active or in standby mode-must be exceedingly low. Third, many possible applications are highly cost-sensitive.
In many such applications, the size and weight of the portable module is extremely sensitive. A module which is merely transportable will not suffice. For example, pagers and portable radios have often had weights of 10 ounces or more, and volumes of 10 cubic inches or more. If modules of this size were used (for example) for patient identification in a hospitals, the patients would unload such cumbersome objects as quickly as possible, by any means possible. Similarly, in many applications such large modules could not be used for inventory control, since there would be no convenient place to put them, and they would easily be damaged (or personnel would learn to bypass them).
In most applications, rechargeable batteries are not suitable for a power supply. Rechargeable batteries not only impose a user burden (to perform recharging), but also tend to have electrical characteristics which may be dependent on the discharge/recharge history of the particular battery. Many possible applications cannot tolerate such uncertainty, and require a degree of reliability which demands a very conservative approach to power supply design and rating.
The need to conserve power actually implies several separate constraints: the consumption requirements of both the active and the standby mode must be separately minimized, and the issues to be considered are somewhat different.
Power conservation also affects the choice of coding scheme. The energy per bit of data successfully sent to the base station, and per bit of data correctly received from the base station, must both be minimized under the conditions actually expected (including distance from remote to base module, RF noise level, and the lengths of data streams which typically need to be handled).
The present application not only discloses significant architectural innovations which provide improved functionality and power efficiency in the portable data module, but also discloses a complete system architecture and coding protocol which provides substantial advantages.
The present invention provides a communications system which uses a variable-symbol-length coding scheme, such as pulse-width modulation or burst-length modulation. In the presently preferred embodiment, information is encoded (in the RF signal) as variable-duration pulses separated by quiet periods. The length of the pulses is thresholded at several levels, so that each pulse can correspond to one of several symbols. The pulse length thresholds are well separated, so that accurate reception will occur even if the receiver makes some errors in measuring the length of the pulses. (In the detection and decoding circuitry, these variable-length pulses are translated into pulse trains at the carrier frequency.)
The coding used maps the most frequently used symbols onto the shortest pulses. Since the direction of information transfer is defined by overhead bits, the same pulse sequence is used to encode a command of "READ" or of "WRITE 0". (Of course, similar benefits could be obtained, alternatively, by combining the "READ" command with the "WRITE 1" command.) In general, the pulse-width code of the presently preferred embodiment attempts to assign the most frequently used commands to the shortest pulses, to maximize the average baud rate. For example, in the presently preferred embodiment, the symbol for "reset" is the next-shortest symbol after the read and write symbols.
Preferred System Context of the Present Invention
In the presently preferred embodiment, the claimed inventions are used in the context of a wireless-accessible data module. Various features of this system context will therefore be described in great detail below. The general features of this context will now be described.
Many of the innovative teachings of the present application will initially be described in the context of an embodiment, as shown in FIG. 1, wherein RF communication is established between a base station and one or more portable data modules. Each portable module can be accessed, in slave mode, by a base station 110 whenever the portable module comes within range of the base station.
A split frequency allocation is used on the RF channel. The base station transmits at a relatively low frequency (referred to herein as the "write-data" frequency), and the remote module transmits at a much higher frequency (referred to herein as the "read-data" frequency). The transmitter powers used permit communication over a very short range.
The portable data module is preferably extremely compact, and is powered by an small non-rechargeable battery. The base station is assumed not to be power-limited, but of course the innovative teachings set forth herein could also be applied to systems where some of the base stations are micropowered and/or some of the portable modules are not micropowered.
Within the remote module, each variable-length pulse in an incoming RF signal is converted into a variable-length burst of digital pulses. These bursts are decoded to derive commands and data. (The portable module operates in slave mode, so that the commands thus received govern its operation.) The portable module also contains an internal serial data bus, and memory or other devices in the portable module can be written to (or read from) over this serial bus, as commanded by the incoming RF signals.
The 3-wire serial data bus within the portable module can be used in a variety of ways. In the presently preferred embodiment, this bus is connected to an access control chip and to the converter chip. In an alternative embodiment, this bus is connected to a memory controller chip (instead of the access control chip), and, through the memory controller chip, to an SRAM. In further alternative embodiments, additional micropowered integrated circuits (such as a microprocessor or display driver) can also be connected to this bus if desired. Similarly, while the portable module preferably also contains access control logic, to provide security against unauthorized access, this can be omitted if desired.
To further conserve power, the portable module has the capability to be completely turned off or on, by wireless control. By keeping the receiver circuit turned off until the module is put into service, the battery life is conserved. In the off state (which is referred to as the "sleep" or "freshness seal" mode, as distinguished from the standby mode), the battery drain is reduced to transistor leakage currents - almost zero power (a few nanoamps). To put the module into service, the whole module is placed in a strong 2 kHz electromagnetic field. A strong coded signal at this frequency is detected by zero-standby-power circuits, with control logic to turn on or turn off all the other detection functions of the receiver.